Diffusion in a series of ionic liquids is investigated by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). It is demonstrated that the mean jump lengths increase with the molecular volumes determined from quantum-chemical calculations. This provides a direct means-via Einstein-Smoluchowski relation-to determine the diffusion coefficient by BDS over more than 8 decades unambiguously and in quantitative agreement with PFG NMR measurements. New possibilities in the study of charge transport and dynamic glass transition in ionic liquids are thus opened.Ionic liquids are under investigation for use as reaction media, in batteries and supercapacitors, solar and fuel cells, electrochemical deposition of metals and semiconductors, protein extraction and crystallization, nanotechnology applications, physical chemistry, and many others.1-3 However, the interplay between the molecular structure and diffusivity in these materials remains unclear despite the fact that diffusion is one of the key processes determining the performance and technological applications involving ILs. In the current study, experimental and theoretical approaches are combined to investigate the quantitative relationship between the structure and dynamics in a series of ionic liquids. For the first time, we demonstrate that the mean ion jump length-a key quantity determining the ion mobility-increases with molecular volume of the ionic liquids investigated.Diffusion is a ubiquitous and fundamental process characterized by the haphazard motion of elementary constituents of matter, most notably of atoms and molecules, due to their thermal energy. It maintains the functionality of living cells, determines the rates of chemical reactions, facilitates electrical conduction, and forms the basis of numerous technological applications.4-6 Fick's first law of diffusion provides a means of explaining the process in terms of mass transport down a concentration gradient. Within this framework, the diffusive flux, j, is given by j ¼ ÀD(c)Vc where c denotes the concentration, D is the diffusion coefficient of the diffusants, and V is a vector del operator. The concentration profile due to diffusion at time t can be determined upon consideration of the principle of mass conservation. This yields Fick's second law of diffusion expressed as vc/vt ¼ V(D(c)Vc). This approach, although widely used, does not provide a direct link to the molecular structure of the material under consideration.Einstein and Smoluchowski proposed a microscopic description of diffusion. According to this view, the particles (diffusants) haphazardly hop, executing random walk quantifiable through the Einstein-Smoluchowski relation (written as hr 2 i ¼ 6Dt, where hr 2 i represents the mean-square distance traversed by the diffusants in time t). The random motion of individual particles gives rise to a diffusive flux on a macroscopic level that can be described by Fick's laws of diffusion. It can be easily shown that the mea...

Transport properties of cyclohexane confined to a silica material with an ordered, bimodal pore structure have been studied by means of pulsed field gradient nuclear magnetic resonance. A particular organization of the well-defined pore structure, composed of a collection of spatially ordered, spherical mesopores interconnected via narrow worm-like pores, allowed for a quantitative analysis of the diffusion process in a medium with spatially ordered distribution of the fluid density for a broad range of the gas-liquid equilibria. The measured diffusion data were interpreted in terms of effective diffusivities, which were determined within a microscopic model considering long-range molecular trajectories constructed by assembling the alternating pieces of displacement in the two constituting pore spaces. It has further been found that for the system under study, in particular, and for mesoporous materials with multiple porosities, in general, this generalized model simplifies to the conventional fast-exchange model used in the literature. Thus, not only was justification of the applicability of the fast-exchange model to a diversity of mesoporous materials provided, but the diffusion parameters entering the fast-exchange model were also exactly defined. The equation resulting in this way was found to nicely reproduce the experimentally determined diffusivities, establishing a methodology for targeted fine-tuning of transport properties of fluids in hierarchical materials with multiple porosities.

Confinement of fluids in porous materials is widely exploited in a variety of technologies, including chemical conversion by heterogeneous catalysis and adsorption separations. Important fundamental phenomena associated with many-molecule interactions occur in such systems, including a remarkably long "memory" of the past when the actual amount of molecules in the pores dramatically depends on the history of how the external conditions have been changed. We demonstrate that the intrinsic diffusivity as measured by NMR serves as an excellent probe of the history-dependent states of the confined fluid. A remarkable feature of our results are differences in diffusivity between out-of-equilibrium states with the same density within the hysteresis loop. This reflects different spatial distributions of the confined fluid that accompany the arrested equilibration of the system in this region.

Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating di †usion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the e †ect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.